This invention relates to the preparation of highly densified ceramic bodies by the pyrolysis of a novel mixture comprising a preceramic borosilazane, silicon carbide powder and, optionally, curing agents for the borosilazane. Such highly densified ceramic bodies can be prepared by sintering under pressure or by utilizing a pressureless process.
Silicon carbide ceramics are well known in the art to have very desirable chemical and physical stability properties at high temperatures. As such, they have found utility in numerous structural applications such as components for aircraft engines and motor vehicles and in various chemical process industries.
Traditionally, silicon carbide ceramics have been made by hot-pressing silicon carbide powder. Although effective, this technique is very expensive because of the high temperature and pressure required. Additionally, it is difficult to create complex shaped bodies by this process.
To avoid these disadvantages, various pressureless sintering methods have been developed. For instance, Prockazka in U.S. Pat. Nos. 4,004,934 and 4,041,117 and Prockazka et al. in U.S. Pat. No. 4,081,284 describe high density silicon carbide ceramic bodies prepared from mixtures consisting of silicon carbide powder, a boron-containing additive, and a carbonaceous additive. Ceramic silicon carbide materials with greater than 85% of theoretical density were obtained.
In addition, various sintering processes using organosilicon polymers are also known. For instance, Onda et al. in Japanese Kokai Patent 60-16869 describe high density silicon carbide ceramics obtained by the pressureless sintering of a mixture of silicon carbide powder, a sintering aid, and an organosilicon polymer characterized by a Si-C skeletal structure. Similarly, U.S. Pat. No. 4,962,069, assigned to the same assignee hereof, discloses sintering mixtures comprising a polysilazane polymer, silicon carbide powder and a sintering aid to form dense bodies.
Takamizawa et al. in German Patent No. 3,500,962 teach the formation of Dense SiC bodies by mixing a polymer having repeating Si-C and B-N bonds with silicon carbide powder and then firing the mixture. This reference, however, does not teach the use of the borosilazanes claimed herein and it does not teach the use of the claimed amounts of ingredients.
The present application discloses for the first time that high density, high strength ceramic products may unexpectedly be obtained by sintering a mixture comprising a preceramic borosilazane and silicon carbide powder.